defer config.add_translation_dirs and push/pop threadlocals during deferred actions/discriminators

[mmerickel]

This resolves #2046.

1. Added docs about importance of config.begin() when invoking add_translation_dirs.

2. Deferred the computation of the specs until the default config phase in order to allow asset overrides defined later to have an effect.

3. Used the AssetResolver to resolve the entire spec instead of just the package name such that translation directories can override each other even if they don't have the same name / subpath.

4. config.override_asset now occurs in an earlier config phase to ensure assets are updated prior to add_translation_dirs executing.

I would appreciate it if @piotr-dobrogost could review this.

[mmerickel]

I will amend this PR tonight to have add_translation_dirs call begin and end itself during the register cycle allowing it to do what it needs to do properly. This will remove the new docs added.

[mmerickel]

Ok I upped the ante and am now calling begin/end during every commit cycle so any deferred action can depend on threadlocals. I think this makes a lot of sense.

This is ready for review.

[mmerickel]

I'm going to merge this tonight unless someone reviews it by then and has an issue. It has global changes to the configurator so I was hoping there might be a review, but I've also added enough tests that I'm pretty confident in it.

[digitalresistor]

Will review tonight.

[digitalresistor]

LGTM.

[digitalresistor]

s/on/only/

[mmerickel]

fixed, thanks

[digitalresistor]

Nice cleanup!

[piotr-dobrogost]

Looks good. Thank you for fixing this.

Ok I upped the ante and am now calling begin/end during every commit cycle so any deferred action can depend on threadlocals.

It would be good to make title for this pull reflect broader scope of the change.